Frequency modulation tone keyer



Aug-22, 1944. M. G, cRosBY I 2,356,224

FREQUENCY MODULATION TONE KEYER Filed July 10, 1942 2 Sheets-Sheet 1 m Q k W m Q g E I a E v INVENTOR it? g M/AWA 6 Gross) QQ "a Lav ATTORNEY Aug. 22, 1944. G RQ$BY 2,356,224

- FREQUENCY MODULATION TONE KEYER Filed July 10. 1942 2 Shets-SheetZ Tlci- INVENTOR Ala/MAY (mass 1."

ATTORNEY Patented Aug. 22, 1944 2,356,224 FREQUENCY MODULATION TONE KEYER Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 10, 1942, Serial No. 450,354

13 Claims.

This application discloses a tone keyer to be used in spaced wave signalling. In spaced wave systems at the transmittera wave is keyed in accordance with signalsfrom one. frequency, say the space frequency, to another frequency, say the marking frequency. This method of signalling is frequency modulation with mark-andspace keying.

My invention concerns a new and improved means for receiving spaced waves from such a transmitter and deriving therefrom the signals. In my system I make use of practically any good frequency modulated wave receiver and adapt the same in a novel manner to the reception of spaced keyed waves. The waves are applied to the receiver andtherein amplified with reduction in frequency, if desired, and fed to the receiver limiter.

The output of the limiter in the receiver is amplitude modulated with a local'tone and fed to the balanced detector usually found in F. M. receivers. In accordance with my invention, the balanced detector is adjusted to balance out the amplitude modulation for the space condition, but allow it to pass for the mark condition or vice versa.

The prior art of tone keyers is confined mostly to the use of on-and-ofi keying, which would normally be called anamplitude modulation type of keying. detected and converted to a direct current which is arranged to bia's an amplifier tube normally biased to cut off to its working point so that it amplifies current fed thereto. The amplifier is fedthelocal'tone current so that the output is a tone which is keyed by the direct current bias in accordance with the keying on the incoming wave".

If such an amplitude modulation tone keying additional apparatus usually required for tone" keying is dispensed with. The system is flexible in that it allows easy switching from reversed to directkeyingnhwnn-. i

In such a system, the keyed wave is In describing my invention reference will be made to the attached drawings wherein:

Figure 1 illustrates a frequency'modulation receiver arranged in accordance with my invention for the reception of spaced wave signals;

Figure 2' illustrates graphically the operation of the system of Figure 1;

Figure 1a is a modification of the arrangement of Figure 1.

Referring to Figure 1, the frequency keyed wave is amplified, reduced in frequency and again amplified in the stage 2 cooperating with oscillation source 4. The-frequency modulated wave is fedthrough transformer H] to limiter 20. Limiter 20 employs the principle described in my U. S. application #275,151, filed May 23, 1939 now U. S. Patent No. 2,276,565 dated. March '17, 1942. Optimum input for best limiting on this tube is indicated by the meter 2! in the plate circuit of the input triode -system of tube 20. The output of the limiter is fed to a frequency discriminator arrangement designated generally at 30.

This discriminator is of conventional type with circuits 3| and 33'off-tuned to either side of the carrier frequency to provide the crossed sloped filter characteristic which converts from frequency modulation to amplitude modulation. The amplitude modulation cancellation is maximum when the carrier is at the crossing point so that the amplitude modulation passed may be represented by the curve C in Figure 2 where F0 isthe crossing frequency of the sloping filters. The detected output of the discriminator appears on the differentially connected resistors 34 and 36 of diodes 40 and 42. v I v The frequency modulated output of the limiter 2|] is amplitude modulated by local oscillator which is fed to the grid of the second triode of the limiter through transformer 52. This oscillator frequency might be 1000 cycles or any other tone frequency suitable for utilization of the keyed output. When this amplitude-modulated frequency-keyed output is fed to the balanced detectors 40 and 42, the amplitude modulation component will be balanced out when the frequency coincides with the crossing point of the discriminator. Thus, by adjusting the spacing frequency F5 to the crossing point of the discriminator, the carrier wave when keyed to the marking frequency .(Fm) will be off the balance point. This is shown in Figure 2 where the am-.

plitude' modulation output of the balanced detectors is plotted against the frequency applied to the discriminator. With the carrier frequency at F0, which is the crossing point of the discriminator, the amplitude modulation, or tone put on at 50, output is zero. When the frequency is keyed to the frequency Fl, the output rises to amplitude at. Thus, When the keyed tone modulated wave shown in the lower part of Figure 2 is applied, the output of the detector is a tone which is keyed between an amplitude of zero and a.

Tube 60 is an audio amplifier which amplifies the detected output and makes it available at jack "Hi. Band-pass filter IE is tuned to the tone frequency and serves the function of eliminating the harmonics of the tone which are present in the output of the detector.

Normally the crossing point of the discriminator is placed in the middle of the band pass of the receiver. However, from the standpoint of the tone keyer, it is more desirable to tune the crossing point of the discriminator over to one side of the receiver band pass shown at BP in Figure 2', so that the full pass of the receiver might be utilized for modulation to thereby improve efficiency of transmission and increase the keyed tone output.

When it is desired to leave the crossing point in the middle of the pass band, for automatic frequency control or other purposes, the circuit of Figure 1a may be used. With this circuit the point at which the detector output is balanced may be shifted to occur with the frequency at one side of the band pass, while the discriminator crossing point is left in the middle of the band pass. The shift of the balance point is produced in Figure 1a by means of potentiometers P and PR which control the'relative outputs from the two detectors 4B and 42. With these potentiometers set forequal attenuation, the detector output balance point coincides With the crossing point of the discriminator. By varying the relative attenuations produced by P and P, the balance point may be shifted from one side of the pass band to the other.

The reason for this is as follows: Assume that the crossing point of the discriminator is in. the middle of the pass band and that the signal is momentarily at one side of the pass band. This will produce a high output of amplitude modulation from, for instance, detector and a low output from detector 42 so that the amplitude modulations no longer balance. It can be seen that balance can be restored for such an off-side condition by increasing the attenuation due to potentiometer P so that thedetected output fed to the push-pull amplifier by that detector is the same as the output from detector 42. The balance is then shifted to one side so that a shift in frequency in the opposite direction upsets the balance again and produces the keyed output. It will also be apparent that by varying the relative attenuations of P and P, the crossing point may be shifted to one side or the other so that the keying may be inverted to produce a spacewhere mark was and vice versa.

Thus, in the modification shown in Figure la the point of crossing of the discriminator circuits' 30 is adjusted to the middle of the receiver band pass. The outputs of the detectors are balanced at a point which coincides with the discriminator crossing point and the differential output across resistances 34 and 33 is used over line AFC and time constant circuit RC to control a reactancetube modulator 5 associated with the beatingoscillator 4 in a well known manner.

However, the input toamplifiers and 4! may the radiating antenna.

be adjusted by P and P so that the balance point does not coincide with the discriminator crossing point and detector output balance point but is at one side of the receiver band pass characteristic.

It will be understood that the tone modulation of the keyed wave may take place at the transmitter. In this case a tone source and modulator of the nature shown here may be introduced as a stage of the transmitter between a point at which the shifted carrier appears and When the tone is put on at the transmitter, the limiter at the receiver is switched out or rendered inoperative as a limiter.

I claim:

1. In a system for demodulating wave energy, the frequency of which is alternately keyed from one frequency to another frequency, and the amplitude of which is modulated in accordance with potentials of a third frequency, a frequency discriminator circuit, of the type having similar but oppositely sloped characteristic curves which intersect, so constructed and arranged that the point of intersection coincides in the frequency spectrum with the frequency of said Wave energy in one of its keyed positions, connections for impressing said wave energy on said discriminator circuit, and balanced detectors coupled to said discriminator circuit.

2. In a system for demodulating wave energy,

. the frequency of which is alternately keyed from band pass of said band pass circuit, connections between said band pass circuit and said discriminator circuit, balanced detectors coupled to said discriminator, and connections for deriving unequal amounts of the outputs of said detectors whereby the effective intersection point is shifted to substantially coincide with the frequency of said wave energy in one of its keyed positions.

3. In a system for demodulating wave energy,

the frequency of which is alternately keyed from. one frequency to another frequency, and the am-' plitude of which is modulated in accordance with potentials of a third frequency, a frequency discriminator circuit, of the type having similar but oppositely sloped characteristic curves which intersect, connections for impressing said wave energy on said discriminator, a pair of balanced detectors coupled to said discriminator circuit, a pair of amplifiers, and adjustable couplings be;

tween the inputs of said amplifiers and outputs of said detectors for applying unequal amounts of the detector outputs to the inputs of said amplifiers whereby the effective intersection point of the system substantially coincides in the fre-v quency spectrum with the frequency of said wave energy in one of its keyed positions.

4. In a system for dem'odulatingwave energy,

the frequency of which is alternately keyed from one frequency to another frequency, an amplitude modulator wherein said frequency keyed wave energy is modulated by potentials of tone frequency to amplitude modulate the wave energy in its keyed positions in accordance with said po tentials of tone frequency, a frequency discriminator circuit, of the type having similarbut op-.

positely sloped characteristic curves which inter sect, so constructed and arranged that the point of intersection coincides in the frequency spectrum substantially with the frequency of said wave energy in one of its keyed positions, and. a pair of detectors having differential output cir-' cuits coupled to said discriminator circuit.

5. In a system for demodulating wave energy, the frequency of which is alternately keyed from one frequency to another frequency, a band pass amplifier for said wave energy, an amplitude, modulator excited by amplified frequencyv keyed wave energy and by potentials of a third frequency to amplitude modulate the wave energy in its keyed positions in accordance with said potentials of a third frequency, a frequency dis: criminator circuit of the type having similar but oppositely sloped characteristic curves which injtersect, and substantially with the middle of the band pass of said amplifier, a pair of detectors coupled to said discriminator circuit and means for deriving unequal amounts of the outputs of said detectors whereby the effective point of intersection substantially coincides in the frequency spectrum with the frequency of said wave energy in one of its keyed positions.

6. The method of adapting a wave length modulated wave receiver having a frequency discriminator circuit, of the type having similar but oppositely sloped characteristic curves which intersect and differential detectors coupled there.-

with, to the reception of wave energy, the frequency of which is alternately keyed from one frequency to another frequency, and the amplitude of which is modulated in accordance with potentials of tone frequency which includes the following steps, adjusting said discriminator and detector so that the effective point of intersection coincides in the frequency spectrum with said wave energy in one of its keyed positions so that the modulations of said third frequency on the wave in said one of said keyed positions oppose and cancel, and deriving the output from said differential detectors.

'7. In a system for signalling with wave energy, the frequency of which is keyed in accordance with signals from one frequency to another frequency and vice versa, a source of wave energy of a third frequency, connections for modulating the amplitude of said keyed wave energy in accordance with said wave energy of said third frequency, a frequency discriminator circuit having an input and having an output, a differential detector system having an input coupled to the output of said discriminator, said detector system having an output, said discriminator having similar but oppositely sloped characteristic curves which intersect, the detector system and discriminator being so constructed and arranged that the said point of intersection of the discriminator and the point of balance of the differential detector system substantially coincides in the frequency spectrum with the frequency of said wave energy in one of its keyed positions, and connections for deriving the differential output of said detector system.

8. In a system for signalling with wave energy, the frequency of which is keyed in accordance with signals from one frequency to another frequency and vice versa, a band pass filter through which said wave energy is passed, a source of wave energy of a third frequency, connections for modulating the amplitude of said keyed wave energy in accordance with said wave energy of said third frequency, a frequency discriminator circuit having an input of which said keyed amplitude modulated wave energy isimpressed and having an output, a differential detector system having an input coupled to the output of said discriminator, said detector system having an output, said discriminator having similar but op positely sloped characteristic curves which intersect, the detector system and discriminator being so constructed and arranged that the point of intersection of the discriminator coincides in the frequency spectrum with the frequency of said wave energy in one of its keyed positions, and the point at which the detector system output is balanced occurs with the frequency at one side of the band pass of said filter, and connections for deriving the differential output of said detector system.

9. In a system for signalling with wave energy, the frequency of which is keyed in accordance with signals from one frequency to another frequency and vice versa, a source of wave energy of a third frequency, connections for modulating the amplitude of said keyed wave energy in accordance with said wave energy of said third frequency, a frequency discriminator circuit having an input excited by said keyed amplitude modulated wave energy and having an output, a balanced differential detector system having an input coupled to said discriminator output said detector having an output, said discriminator having similar but oppositely sloped characteristic curves which intersect, the detector system and discriminator being so constructed and arranged that the point of intersection of the discriminator substantially coincides in the frequency spectrum with the frequency of said wave energy in one of its keyed positions and the demodulation components corresponding to amplitude modulations on the said wave energy of said one frequency oppose and cancel in the detector system output, and connections for deriving the differential output of said detector system.

10. In a system for signalling with wave energy, the frequency of which is keyed in accordance with signals from one frequency to another frequency, a band pass filter through which said wave energy is passed, a source of wave energy of a third frequency, connections for modulating the amplitude of said keyed wave energy in accordance with said wave energy of said third frequency, a frequency discriminator circuit having an input excited by said keyed amplitude modulated Wave energy and having an output, a differential detector system having an input coupled to said output of said discriminator, said detector system having an output, said discriminator having similar but oppositely sloped characteristic curves which intersect, the detector and discriminator being so constructed and arranged that the point of intersection of the discriminator and the balance point of the detector system substantially coincide and occur at one side of the frequency spectrum passed by said band pass filter, and connections for deriving the differential output of said detector system.

11. In a system for demodulating wave energy, the frequency of which is alternately keyed from one frequency to another frequency, a band pass amplifier for said wave energy, a source of wave energy of a third frequency, an amplitude modulator associated with said source of wave energy of said third frequency and excited by said keyed wave energy to amplitude modulate the wave energy in its keyed positions in accordance with said wave energy of said third frequency, a frequency discriminator circuit having similar but oppositely sloped characteristic curves which intersect at a .point which substantially coincides in the frequency spectrum with the'frequency of said Wave energy in one of its keyed positions and falls at one side of the bandpass of said filter, and a pair of balanced differential detectors coupled to said discriminator circuit, said detectors having opposed outputs wherein the demodulated amplitude modulation on said wave energy in its said one keyed positions cancel.

12. In a system for demodulating Wave energy, the frequency of which is alternately keyed from one frequency to another frequency, a band pass amplifier for said wave energy, a source of wave energy of a third frequency, an amplitude modulator associated with said source of wave energy of said third frequency and excited by said keyed Wave energy to amplitude modulate the wave energy in its keyed positions in accordance with said wave energy of said third frequency, a frequency discriminator circuit, having similar "but oppositely sloped characteristic curves which intersect at a point in the frequency spectrum which substantially coincides with the center of the band pass of said-band pass filter, a pair of differential detectors having inputs coupled to said discriminator circuit, said differential detectors having output circuits wherein the amplitude modulation, derived from demodulating said by said keyed wave energy to amplitude modulate the wave energy in its keyed positions in accordance with said wave energy of said third frequency, a frequency discriminator circuit having similar oppositely sloped characteristic curves which intersect at a point which substantially coincides in the frequency spectrum with the center of the band pass of said band pass filter, a pair of balanced differential detectors coupled to said discriminator circuit, an automatic frequency control circuit coupled to the output of said balanced detectorsfor stabilizing the frequency of said wave energy, and a differential amplifier system with a non-symmetric input coupled to said differential detectors.

MURRAY G. CROSBY. 

